1. Field of the Invention
This invention relates to an industrial truck with a lifting height measurement system and a method for measuring lifting height on an industrial truck of this type. The invention relates in particular to an industrial truck with a lifting mast and vertically adjustable load handling means located on the lifting mast, wherein the lifting mast has a lifting height measurement system having of a light emitter that emits a light beam, a receiving element, an optical alignment element, a target element, and a measurement path located between them, the length of which measurement path changes with the lifting height.
2. Technical Considerations
Various lifting height measurement systems are known on industrial trucks with a lifting mast or lifting mast on which a load handling device, most frequently a load fork, is guided in a vertically adjustable manner for the transport of the load. The measurement systems make it possible to measure the distance by which the load handling device of the industrial truck is raised. For example, cable pull sensors are used, by means of which the length of a cable paid out from a cable roller or pulley when the load handling device is lifted can be measured. Other systems use a cable to which vertical tension is applied and which is wrapped around a measuring pulley, so that this measuring pulley is moved when the load handling device is moved upwardly and is placed in rotation by the cable that is wrapped around it. Conclusions about the height of the load handling device can be drawn by measuring the rotational movement.
One problem with these solutions is that the cables used, or the belts that are sometimes used as an alternative, often break because they are subject to mechanical wear and experience material fatigue because of the alternating stresses applied to them, or they can become snagged on objects such as shelves, loads etc.
A desirable objective for a lifting height measurement system is an absolute measurement of the lifting height instead of an incremental measurement which measures a relative change in the lifting height, and the actual total lifting height must be calculated continuously. An absolute measurement prevents cumulative errors in this continuous determination of the actual lifting height.
Under these conditions, the use of a laser distance sensor is attractive because a laser distance sensor performs an absolute height measurement. Because of its small size it can be easily installed in an industrial truck in the vicinity of the lifting mast and also does not require the routing of any lines or cables through the lifting mast.
FIGS. 2-5 illustrate various prior art embodiments of lifting height measurement systems.
However, the use of a laser distance sensor to perform a measurement, such as, for example, a laser sensor located in the lower portion of the industrial truck which directs a laser beam upwardly toward a target element, such as a suitable reflective target, for example, leads to problems because the position of the target, e.g. when a load is being transported or when the industrial truck accelerates/decelerates, can change significantly when the vehicle is in motion, because such a motion is accompanied by a bending of the lifting mast, e.g. out of its vertical axis, and the target moves out of the beam path of the laser distance sensor in the direction of the longitudinal axis of the vehicle or laterally with respect to the vertical axis at a right angle to the longitudinal axis of the vehicle.
When a laser distance sensor of the known art is used, the only effective countermeasure for this problem is to make the horizontal surface of the target correspondingly large. However, this requirement cannot always be met given the limited space available in the lifting masts of industrial trucks.
An additional problem that occurs with lifting height measurement systems on industrial trucks and that must be taken into consideration is the fact that as a result of wear on the tires, the ground clearance of the vehicle decreases with time and distorts the absolute value of the lifting height, which must be determined very accurately to avoid a collision with a shelf, for example. With super-elastic tires in particular, wear leads to significant changes that can be in the vicinity of a centimeter.
EP 1 866 966 A2 describes a device for contactless measurement of the lifting height in which a transmitter device transmits a wave-form signal, in particular an electromagnetic wave, to a separately located receiver device. The transmitter and receiver devices are located on moving components that move in relation to each other during lifting movements of the lifting mast. One disadvantage of this device is that both the receiver as well as the transmitter device require signal lines and a power supply, all of which must be taken into consideration accordingly in terms of their location and arrangement in the lifting mast.
DE 10 2008 020 170 A1 describes a method for measurement of the height of vertically adjustable load handling means of an industrial truck. In this method, an image of a target that is movable relative to a receiver device is registered by the receiver device and the position of the load handling means is determined from the change of the image.
It is an object of the present invention to provide an industrial truck with a lifting height measurement system which is suitable for use on an industrial truck and can be integrated into a wide variety of lifting masts with little effort or expense.